Method and apparatus for geophysical exploration using GMR sensors

ABSTRACT

An instrument for geophysical measurement of magnetic field strength, using a GMR sensor is provided. The instrument achieves high sensitivity across a relatively wide bandwidth in an instrument which is small and lightweight. The instrument makes it feasible to obtain data, substantially simultaneously, at a plurality of locations and/or frequencies, thus not only reducing time requirements involved in measurements but reducing or eliminating the need for correcting data for changes in earth&#39;s magnetic fields. The instrument has relatively high vector sensitivity and relatively low power consumption.

The present invention is directed to a method and apparatus for a giantmagneto resistance (GMR) based instrument and in particular to a GMRinstrument for geophysical exploration.

BACKGROUND INFORMATION

A number of instruments are used in connection with measuring magneticfields for the purpose of geophysical exploration, such as for locatingregions likely to contain minerals, petroleum, water, natural gas,hazardous materials or other items or materials of interest. In general,geophysical exploration instruments for detecting magnetic fields can beconsidered in two categories: "passive" systems for measuring a magneticpotential field (near-zero frequency field), sometimes referred to asmagnetometers, and active field instruments in which a transmitter emitselectromagnetic radiation (periodic or other time-varying radiation)which travels through a portion of a geologic formation and is detectedat a second location by a receiver or detector instrument.

Previous potential field instruments and active field instruments havetypically been cumbersome to properly locate in the field, for takingdesired data. In some cases, instruments are cumbersome because of theirphysical size or mass, often involving relatively large coils which,previously, were believed necessary in such instruments in order toattain the desired sensitivity. In instruments intended for boreholemeasurements, size is particularly important because of the strongrelationship between borehole diameter and cost. Many instruments usedfor borehole measurement of magnetic field strength were relativelyfragile, such as those based on a tuned ferrite rod.

Some previous instruments require special environments such as cryogenicenvironments or elaborate calibration or initialization procedures. Suchprevious instruments requiring cumbersome positioning or setup have,therefore, required a substantial investment in personnel time for eachdata-gathering section. In many exploration techniques, it is necessaryto obtain data at a plurality of spaced-apart locations (such as anumber of points along a line or a number of points in a grid). Theinstrument cost or personnel cost may make it infeasible to position aplurality of instruments simultaneously along the line or grid and thusone or few instruments are positioned and, after taking the desired datamoved to another location along the line or grid. Thus, the personneltime requirements for taking data in an extensive region can berelatively high.

In many previous systems, separate instruments were needed for measuringa potential field (near-zero frequency field) and measuring anelectromagnetic field (time-varying field), each instrument typicallyrequiring its own cumbersome placement or setup procedures.

Generally, collecting data at more than one frequency can be desirablesince location of targets is facilitated depending on the target sizeand depth below surface (lower frequencies being used for deeper orlarger targets) and since the response for given target may be differentat different frequencies for other reasons as well (e.g., targetcomposition, and physical conditions of temperature, pressure and thelike). Many previous instruments (including, many coil instruments,typically used for surface-based measurement and ferrite rod sensorsused, e.g. for borehole measurements) were configured to sense only asingle frequency or relatively small band of frequencies. Otherinstruments were configured to sense a plurality of different discretefrequencies (or relatively small frequency bands), but, typically, couldsense only one frequency (or frequency band) at a time, (i.e. wereincapable of sensing multiple frequencies substantially simultaneouslyand were incapable of sensing a frequency continuum or wide band widthof frequencies). Thus, in many previous systems, in order to obtain dataat a plurality of frequencies, it was necessary to take data at a firstfrequency during the first time period, reconfigure or substituteinstruments and then take data at a second frequency at a later time.This characteristic of such instruments further contributed torelatively large personnel time requirements for geophysicalexploration.

An additional reason for desirability of wide bandwidth sensitivity forgeophysical instruments relates to collection of active field data. Fordata analysis purposes, the source (transmitter) waveform is oftenassumed to have an idealized form, such as the square wave signaldepicted in FIG. 6A, defining a period T 612 and, thus, a frequencyf=T⁻⁰.5. Although FIGS. 6A and 6B depict time-varying voltage signals,the source signal can also be a time-varying current signal, withappropriate changes in the receiving instrument. In order to detect asignal having frequency f, the (typically fixed) sample frequency f_(s)must comply with the Nyquist criterion:

    f.sub.s ≦2f.

Preferably, the sample frequency will be about 20 to 50 times the sourcesignal frequency.

In practice it is typically not feasible to provide a square wave sourcesignal, as depicted in FIG. 6A, and in most cases, the source signalwill be more similar to the waveform depicted in FIG. 6B, having afinite falling or edge transition period t_(f) which thus defines atransition frequency f_(t) ≈0.35 (t_(f))⁻⁰.5. Previous approaches hadinsufficient bandwidth to recover certain information, and typicallyperformed data analysis by treating the source signal as if it were anidealized square wave signal (e.g. by ignoring the transition period).Such previous approaches thus failed to recover certain potentiallyuseful information. In order to recover the integrity of the sourcesignal of FIG. 6B, the detection instrument needs a system frequencyresponse which can recover both f and f_(t). Typically, previouscoil-based receivers have had insufficient bandwidth to detect bothfrequencies substantially simultaneously.

The above-noted relatively high personnel time requirements associatedwith previous instruments not only adds to the cost and delay ingeophysical measurements but can affect the quality of the data and thetime needed for data processing. This is because the earth's ambientmagnetic field in a given location (which represents a background signalto the signal being measured) can vary significantly in the time frametypically required to complete a series of measurements using previousinstrumentation. For example, if data is collected at a plurality oflocations along a line, at a plurality of times during the day, it mayrequire significant post-collection data processing to discriminatebetween changes in potential fields or electromagnetic fields arisingfrom geophysical items of interest and those that result from thenatural variation of the earth's magnetic field during the day. The samedata discrimination problem is faced with regard to measurements atdifferent frequencies which, as noted above, typically must be collectedat different times. Moreover, even with sophisticated data processingtechniques, it is not always possible to reliably discriminate betweenfield changes arising from geophysical phenomena and those arising fromvariations in the earth's magnetic field.

In many previous systems, it was difficult, expensive and/ortime-consuming to obtain vector information. For example, althoughcoil-type detectors can have some degree of vector sensitivity, theyhave been generally difficult to deploy as two or three vectorreceivers.

Many previous systems had relatively high power requirements for thereceivers or sensors. Such high power requirements added to thedifficulty of obtaining field data since a large power source had to beprovided in often remote locations.

Accordingly, it would be useful to provide a geophysical sensor forpotential fields or active electromagnetic fields which are relativelyless cumbersome and costly to provide, locate and setup, but are,preferably, non-fragile and relatively rugged, have good vectorsensitivity, relatively low power requirements, have a relatively widebandwidth, can measure both "passive" potential fields and activeelectromagnetic fields, preferably simultaneously, have a relativelyhigh sensitivity and/or are feasible for collecting data at a pluralityof locations and/or frequencies, or across a frequency range,substantially simultaneously.

SUMMARY OF THE INVENTION

The present invention includes a recognition of the existence and natureof problems in previous approaches, including those summarized above.The present invention involves geophysical instrumentation using asensor, or an array of sensors, which are based on the so-called giantmagnetoresistive (GMR) effect. Preferably, sensors are provided having arelatively high sensitively (such as a sensitively of about ±2microOersted (μOe), and preferably a relatively high bandwidth, such asa bandwidth from about 0 Hz (DC) to about 4 MHZ or more.

In one embodiment, one or more sensors are positioned and potentialfield data and/or active field data are collected at a plurality offrequencies and/or locations, preferably substantially simultaneously.In one embodiment, a plurality of sensors are positioned at spaced-apartlocations along a line or path which may be substantially horizontal(e.g. along the earth's surface) or substantially vertical (e.g. withina bore hole), or a plurality of sensors may be positioned at pointsacross a two-dimensional region such as points of a grid defined acrossa region of the earth's surface. Transmitters of an active system may bepositioned or located in two or more different configurations in orderto achieve three-dimensional data (e.g. using triangulation methods).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is block diagram of a detector instrument according to oneembodiment of the present invention;

FIG. 1B depicts an equivalent network of a GMR sensor;

FIG. 2 is a schematic depiction of a power source portion of aninstrument according to an embodiment of the present invention;

FIG. 3 is a schematic depiction of a power conditioning portion of aninstrument according to an embodiment of the present invention;

FIG. 4 is a schematic depiction of a sensor, amplifier and filterportion of an instrument according to one embodiment of the presentinvention;

FIG. 5 is a top plan depiction of a two-dimensional sensor arrayaccording to a embodiment of the present invention;

FIG. 6A is a diagram of an idealized source signal showing voltageversus time,

FIG. 6B is a diagram of a period of a source signal with an exaggerateddepiction of the transition edge, showing voltage versus time;

FIG. 7 is a block diagram of a detector instrument according to anotherembodiment of the present invention; and

FIG. 8 is a schematic depiction of a sensor, amplifier and filterportion of an instrument according to the embodiment of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, according to one embodiment, a detecting instrument100 includes a power source 200 (an example of which, according to oneembodiment, is depicted in greater detail in FIG. 2), power conditioning300 (an example of which, according to one embodiment, is depicted inmore detail in FIG. 3) and a sensor, amplifier and filter 400 (anexample of which, according to one embodiment, is depicted in greaterdetail in FIG. 4). The filter at the output is optional, and, asdepicted, is used for anti-aliasing. When such a filter is used, it willtypically restrict the bandwidth of the instrument, such that theinstrument bandwidth will be less than the sensor bandwidth.

An important component of the instrument in FIG. 1 is the giant magnetoresistance (GMR) sensor 112. Although GMR sensors have been developedfor various applications such as use in computer disk storage devices,vehicle guidance (e.g. for "smart highway" applications) and the like.GMR sensors developed for such other applications typically are intendedto detect greater field strengths (compared to those detected ingeophysical exploration applications) where high sensitivity can bedetrimental. Sensors intended for such applications typically have hadinsufficient sensitivity to be suitable for most geophysical explorationapplications. In particular, a previous GMR sensor typically had asensitivity of, for example, about ±15 μOe. In order to be used for mostgeophysical exploration applications, it is believed that the GMR sensor112 should have a sensitivity of about ±50 μOe, preferably about ±10μOe, more preferably about ±5 μOe, even more preferably about ±2 μOeand, most preferably, about ±1 μOe.

Furthermore, for most geophysical exploration applications, in order tosolve problems associated with previous approaches, it is believed thatthe GMR sensor 112 should preferably have a relatively large bandwidthof frequency response. Although, in some embodiments, the sensor mayprovide a bandwidth from substantially DC to 10 kHz or 50 kHz, in otherembodiments, an even larger bandwidth is provided, such as a bandwidthof from about 0 Hz (DC) to about 100 KHz, preferably up to about 4 MHZ,more preferably up to about 50 MHZ and even more preferably up to about100 MHZ.

Moreover, in order to solve problems associated with the previousinstruments, it is believed preferable to provide an instrument in whichthe GMR sensor 112, and associated circuitry is relatively small,light-weight, rugged (non-fragile), requires no special environment(e.g. does not require a cryogenic environment) and requires little ifany calibration or setup. Although sensor and/or instrument mass willdepend on a number of factors, such as the particular application andthe packaging used, in one embodiment, the sensor and associatedcircuitry have a mass, for a one-vector instrument, less than about 0.5kg, preferably less than about 100 g, and more preferably less thanabout 50 g, and occupy a volume less than about 500 cm³, preferably lessthan about 80 cm³, preferably positionable in a bore hole with adiameter of about 2 inches (about 5 cm), and operates at a temperatureabove about 80° K, preferably above STP, more preferably above 70° C.,more preferably 150° C., and even more preferably above about 250° C. orhigher.

Although power consumption is affected by a number of factors, includingsensitivity, in one embodiment, the sensor or receiver instrument hasrelatively low power consumption, such as less than about 300 milliwattsfor a three-vector instrument, preferably, less than about 80milliwatts. Such low power consumption is particularly significant whencompared to instruments which need to be cooled, such as requiringcryogenic cooling, which typically has relatively large powerconsumption. For these purposes, a comparison may be made based on thetotal power consumption of the data collection system (including, forprevious systems where it was used, cooling power, but not including thedata collection/display computer or other data device), divided by thenumber of sensors in the system. For the present invention, theper-sensor system power consumption is preferably less than one Watt,preferably less than 100 milliwatts.

Preferably, the sensor or receiver instrument has relatively high vectorsensitivity, such as achieving unwanted signal rejection of about 20 dB,preferably, about 30 dB, more preferably about 50 dB and even morepreferably about 60 dB or more. Because the sensor is relatively small,it is feasible to place, e.g. three orthogonally-oriented sensors in asingle instrument to provide for collecting 3-vector data substantiallysimultaneously.

Certain characteristics of a suitable GMR sensor are described, e.g., in"The Use of Giant Magnetoresistance Technology in ElectromagneticGeophysical Exploration" by T. David McGlone, published in TheProceedings of the Symposium on the Application of Geophysics toEngineering and Environmental Problems (SAGEEP), March 1997, Vol 2, p705-713, sponsored by the Environmental and Engineering GeophysicalSociety (EEGS) and "The Application of Thin-Film Magneto-ResistiveStructures to a Non-Inductive Receiver for Electromagnetic GeophysicalExploration" by T. David McGlone, Colorado School of Mines, December,1996, both of which are incorporated by reference.

The GMR structure consists of alternating layers of ferro- andpara-magnetic materials. In an illustrative example, layers ofparamagnetic copper (Cu) and ferromagnetic cobalt (Co) are assumed.

It is a necessary condition that the individual layer thicknesses beless than the mean free travel path length of the electron. This pathlength is essentially the distance a free electron travels before ascattering event. In practice, individual layer thicknesses on the orderof 10 Å are typical. Because of the physical size of this structure, itis reasonable to make the assumptions that collisions only occur at thelayer interface where there is a discontinuity in the lattice structureand that each individual ferromagnetic layer consists of a singlemagnetic domain.

With no external field, the intrinsic fields of the ferromagnetic layersforces each alternating ferromagnetic layer to be aligned anti-parallel.As the external field strength increases, the relative orientations ofthe individual ferromagnetic layer domains change until some saturatingfield strength is applied and the individual layer domains becamealigned in a parallel orientation. The primary role of the paramagneticlayer is to separate the ferromagnetic layers so that the anti-parallelfield orientations between the ferromagnetic layers can occur.

Consider the electron quantum property of magnetic spin moment. This isa binary state representing the spin direction of the electron where thestates are designated as "up" and "down". These terms are not related tospatial orientation and the property of magnetic spin moment isconsidered conserved during a scatter event for this discussion.

A metallic conductor typically has a cubic crystalline latticestructure. Using Cu as an example, the metallic atom has three filledand assumed stable inner electron orbits with a single electron in theouter fourth orbit. The ease with which the outer electron is brokenfrom its host atom is the property which makes Cu a good conductor.

The lattice may be pictured as a rigid structure consisting of the innercores of Cu atoms with clouds of free electrons from the outer shellstraveling within the lattice. The outer electrons are in an agitatedstate; continuously breaking free from an atom (thereby leaving a "hole"behind) and traveling within the lattice until it is re-captured byanother atom. The quantum nature of the electron prevents capture by anatom not having a hole with matching characteristics. This requiredmatching of the magnetic moment properties is fundamental to the GMReffect.

In typical paramagnetic conductors, there is an approximately equaldistribution of up and down carriers and available holes for them sothat small variations in magnetic fields have a negligible effect on theconductivity of the conductor. In ferromagnetic materials, there is anet magnetic moment and an unequal distribution of up and down stateelectrons. A free minority-state carrier will take longer to find anavailable hole than will a free majority-state carrier within a singlemagnetic domain.

Within the GMR structure, electron scattering occurs primarily at thelayer interface where a break in the lattice structure occurs. When afree carrier approaches the interface region, the intrinsic magneticfield of the ferromagnetic layer will affect the scattering angle of thecarrier. A favored state electron will be scattered in a favoreddirection having high conductivity while the unfavored state electronwill be scattered in an unfavored direction having low conductivity.Because of the anti-parallel orientation of alternating ferromagneticlayers, the favored state at one layer is the unfavored state in theadjacent layer.

As an increasing external magnetic field is applied, a point ofsaturation occurs in which the intrinsic field holding the layers inanti-parallel orientation is overcome and the alternating ferromagneticlayers become aligned in a parallel orientation with each other. In thiscondition, there is one favored state of carrier for the full structure.If there is an unequal distribution of carrier states unbalanced in thefavored state, a structure of parallel low resistances occurs.

An electromagnetic receiver was built based on this technology toexplore the feasibility of using these structures in geophysicalexploration. The GMR structure has a stated but unverified bandwidthresponse of DC to 50 MHz and a field sensitivity of 5 pT. Provisions aremade for nulling of the ambient magnetic field to increase instrumentsensitivity to the field perturbations of interest. While not asaccurate as SQUIDs and other near-DC magnetometers at low frequencies,the sensor itself is of negligible size, operates at ambienttemperatures and has a vector-sensitive response over a wide bandwidth.This allows the possibility of both frequency and time domainmeasurements to be collected with the same instrument. The transmitterused for both the laboratory and field characterization consisted of amagnetic dipole generated by current-carrying coils.

In one embodiment, the GMR sensors 112 can be sensors such as thoseavailable from Nonvolatile Electronics of Eden Prarie, Minn.

As shown in FIG. 1A, the GMR sensor 112 receives excitation voltage atpoint A 124 using power source 200 and power conditioner 300.Preferably, all connections made to the Single Point Ground 116 (whichis physically and electrically the same as the point 116' shown adjacentthe anti-aliasing filter 416) are physically short and of low impedance.The V-I converter 118 holds point B 126 of the GMR sensor 112 to anominal 0 volts. The symmetry of the GMR sensor will force point C 128to also be at a nominal 0 volts (plus the desired, but small, signalvoltage). This reduces the common-mode voltage restrictions on thecircuit. The no-field voltage at point D 122 is ideally the inverse ofthe excitation voltage.

The output of the VI converter 118 drives the potential of point D 122of the GMR sensor 112. Because active element number two 142 of the GMRsensor equivalent network 144 (depicted in FIG. 1B) is in the feedbackpath of the V-I converter 118 (between points B 126 and D 122), theoutput voltage of the V-I converter at point D 122 varies with theapplied magnetic field to be sensed. This variable voltage is reflectedback to GMR sensor point C 128. Active element number one 146 of the GMRequivalent network (FIG. 1B) between points A 124 and C 128 is alsoactive. Because point A 124 has a fixed potential driven by theexcitation voltage generator, the variation in active element number one146, due to a change in applied magnetic field, is also reflected topoint C 128. The potential voltage at point C 128 is amplified andfiltered before being output to a data display or recording system 114which may be, for example, a computer, such as a personal computer,workstation computer or the like.

As shown in FIG. 2, a power supply 200 receives supply voltage 212a,212b of between ±16 VDC and ±29 VDC and outputs voltages of, in thiscase +15 V and -15 V 214a, 214b with respect to ground 216.

As shown in FIG. 3, the power conditioning circuit 300 receives thisvoltage 214a, 214b and outputs excitation voltage 312a, 312b from avoltage generation 314 and low-pass (≦about 3 to 10 Hz) 316 filter.Filter 316 is useful in reducing high frequency noise to providesubstantially noise-free excitation voltage through the sensor circuit400. A current boost 318 is provided in order to reduce loading of theexcitation voltage generator while maintaining a precise excitationvoltage. A power preregulation 322 is provided in order to provide arelatively noise-free and stable voltage source to the excitationvoltage source. Power supply variations and noise injected into thesensor become indistinguishable from a low level signal. Bypre-regulating the source voltage, and low-pass filtering the sourcevoltage, power supply variations and noise may be reduced, e.g. by about75 dB or more, before encountering the excitation voltage generatorwhich further reduces noise and fluctuations, e.g. by an additional 105dB.

As can be seen from FIG. 4, the sensor circuit 400 is configured as aWheatstone bridge which is, however, forced to a current mode ratherthan a voltage mode. The op-amp 414 is strapped across one leg of thebridge resulting in substantially linear response. The non-invertinginput of the op-amp 414 is strapped to ground in order to eliminatecommon mode voltages.

FIG. 7 shows another embodiment of the present invention. Similarly tothe embodiment of FIG. 1, the GMR sensor 112 receives excitation voltageprovided using a power source 200 and power conditioner 300. Output fromthe GMR sensor 112 is preferably amplified and filtered 800 before beingoutput to a data display or recording system 114 which may be, forexample, a computer such as a personal computer, workstation computer orthe like.

As depicted in FIG. 8, the excitation voltage 312a, 312b is provided tothe sensor 112 which outputs signal voltage 812a, 812b proportional tothe strength of an applied magnetic or electromagnetic field. Thisvoltage is applied to an amplifier such as op-amp 814. The output fromthe op-amp 814 is provided to an anti-aliasing filter 816 before beingoutput to the data display and recording system 114.

In use, at least one and preferably a plurality of instruments such asthose depicted in FIGS. 1-4 are located at a plurality of positions512a-512t along a path or across an area (FIG. 5). In one embodiment,the instruments are miniaturized (such as being embodied in one or moreintegrated circuits or chips) and a plurality of such instruments arecoupled to a length of one or more signal wires 514a-d and positionedsuch as by unrolling a coiled sensor wire in the desired location. Thesensor wires 514a-d are coupled to a data display/recording device 114and excitation voltage is supplied to the sensors while the datadisplay/recording device outputs or records data indicative of thepotential field at the plurality of instrument locations 512a-512t,substantially simultaneously.

In situations in which active field data is desired, one or moreelectromagnetic field transmitters (of types well known in the art) arepositioned at a location 516a spaced from the sensor locations 512a-t.The sensors each output data indicative of electromagnetic fieldstrength at a plurality frequencies, preferably across a substantiallycontinuous spectrum of frequencies and, preferably, substantiallysimultaneously with measurement of the potential field (near-zerofrequency) strength.

When three-dimensional information is desired, the transmitter may berepositioned at a new, preferably orthogonal (with respect to the sensorarray) location 516b and the resultant data combined with the data fromthe originally-located transmitter, using well-known triangulationmethods to deduce three-dimensional field strength data.

As seen from the above description, since it is feasible, using thepresent instruments to obtain data substantially simultaneously at anumber of locations and/or at a number of frequencies (or over arelatively wide frequency bandwidth) all of the data necessary for thedesired geophysical exploration can be obtained in a relatively shortperiod of time, resulting not only in reducing the personnel timerequired for placement, setup and data collection but also reducing oreliminating the need to correct for changes in the earth's magneticfield, since the required measurements can be obtained over a timeperiod short enough that changes in the earth's magnetic field arerelatively insignificant. The wide detection frequency bandwidth whichis possible with the present invention makes it feasible to make use ofthe transition period by measuring its frequency f_(t).

In light of the above description, a number of advantages of the presentinvention can be seen. The present invention provides an effectivegeophysical sensor which is relatively small, lightweight, low-cost,rugged and thus relatively easy and quick to locate in multiplepositions, either successively or simultaneously. The present sensorrequires no special temperature or other operating conditions andrequires little if any calibration or other setup. The present sensor isable to obtain both passive (near-zero frequency) and active(electromagnetic field) measurements, substantially simultaneously,using a single instrument. The present invention is able to achieverelatively high measurement sensitivity at relatively low instrumentcost and high measurement convenience. The present invention providesgood vector sensitivity. The present invention can be configured toprovide relatively low power consumption. The present invention canprovide data which requires a significantly reduced amount ofpost-collection data processing, and in particular can eliminate orreduce the need for corrections as a result of changes in the earth'smagnetic field. The present invention has a relatively wide bandwidthallowing measurement at multiple frequencies and, preferably, across asubstantially continuous spectrum of frequencies, substantiallysimultaneously, without changing or reconfiguring the instrument.Preferably, the instrument has sufficient bandwidth to detect both theprimary frequency f and transition frequencies f₁ (FIGS. 6A and 6B).Furthermore it is noted that, to reconstruct the sine wave equivalentfrequency f, it is necessary to sample at least twice during the sourcesignal period T. Preferably, the instrument of the present invention hassufficient bandwidth that, by dynamically changing the sample periodduring the transition period t_(f), both the frequency domain signal fand the time domain signal f_(t) can be detected. It is believed thatprevious approaches, particularly those using coils for detectionsensors, had insufficient bandwidth to recover both f and f_(t).

A number of variations and modifications of the invention can also beused. The present invention can be configured with up to three (or more)components and configured to detect up to three dimensions, depending onthe set-up of the geophysical survey and version of the instrument. Forexample, the invention can be configured as a three-component,two-dimensional and/or three-dimensional electromagnetic array system.The present invention can be configured to achieve joint frequency andtime domain analysis. Although the depicted embodiment is configured torespond to a time-varying voltage source, the invention can beconfigured to respond to a time-varying current source. Although thedetection instrument provides a voltage mode output, a current modeoutput can be used. Although the depicted embodiment is configured withsingle-ended output, the invention can be used in connection with adifferential output. The present invention can be configured toaccommodate borehole measurement applications such as frequency-tunableborehole measurements. The instrument's amenability to configuration ina small, easily portable size makes the instrument useful for a numberof purposes and in a number of modes of deployment. For example, theinstrument may be deployed as a static installation, such as forenvironmental monitoring of holding ponds for waste products, in amobile use, such as allowing a user to walk with the instrument an takea measurement at selected locations (e.g. by pushing a button). Theinvention can be used in connection with metal detectors, includingdetectors for use by hobbyists (e.g. by configuring the instrument withcurrent mode output for driving a center-null indicator). The inventioncan be used in connection with ground-based vehicle monitoring (e.g.where measurements are taken from a truck or the like), as well asdeployment in air- or spacecraft, e.g. depending on the desired traffic.The present invention can be configured to achieve relatively shallowtime-domain measurements such as on the order of about 20 nanoseconds.The present invention provides sufficient detection frequency bandwidthto permit use in connection with complex or sophisticated sourcesignals. Wide bandwidth allows feasibility of exotic signal sources suchas chirp sources (similar to those used in radar processing). The squarewave signals previously discussed may define a signal envelope. Forexample, a 1 Hz square wave may define an envelope within which a sinewave is swept e.g. from 100 Hz to 1 kHz, allowing a quasi-continuousspectrum to be measured over the swept frequency range.

Although the present invention has been described by way of a preferredembodiment and certain variations and modifications, other variationsand modifications can also be used, the invention being defined by thefollowing claims.

What is claimed is:
 1. A geophysical exploration system for detectingmagnetic fields adjacent a subterranean target region, comprising:aplurality of detectors, each detector having at least one GMR sensorcoupled to an excitation circuit, said detectors positioned atspaced-apart locations on or below the earth's surface; andcommunication links conveying output from said detectors to a datacollection device.
 2. A system, as claimed in claim 1, wherein saidcommunication links comprise cables.
 3. A system, as claimed in claim 1,wherein said data collection device comprises a computer.
 4. A system,as claimed in claim 1, wherein said plurality of detectors arepositioned along a substantially one-dimensional path.
 5. A system, asclaimed in claim 1, wherein said plurality of detectors are positionedacross a two-dimensional region.
 6. A geophysical exploration system fordetecting magnetic fields adjacent a subterranean target region,comprising:a plurality of detectors, each detector having at least oneGMR sensor coupled to an excitation circuit, said detectors positionedat spaced-apart locations on or below the earth's surface, each saiddetector having a mass of less than about 0.5 kg; communication linksconveying output from said detectors to a data collection device; saiddetectors operating above about 70° C., providing a sensitivity ofbetter than about ±10 microOersted and a bandwidth from substantially DCto about 4 MHZ and having sufficient vector sensitivity to provide arejection of at least about 30 dB.
 7. A method for geophysicalexploration comprising:providing at least a first GMR sensor; locatingsaid first sensor at a first location; measuring a magnetic fieldstrength at said first location using said first sensor, indicative of ageophysical characteristic of a subterranean region through which saidmagnetic field passes.
 8. A method, as claimed in claim 7, wherein saidstep of measuring includes measuring a potential field and atime-varying field strength substantially simultaneously.
 9. A method,as claimed in claim 7, wherein said step of measuring includes measuringat a plurality of frequencies substantially simultaneously.
 10. Amethod, as claimed in claim 7, wherein said step of measuring includesmeasuring at a continuum of frequencies between about 0 Hz and about 4MHz.
 11. A method for geophysical exploration comprising:providing aplurality of GMR sensors; locating said plurality of sensors at aplurality of locations, measuring a magnetic field strength at saidplurality of locations, substantially simultaneously, using saidplurality of sensors.
 12. A method for geophysical explorationcomprising:providing a plurality of detectors, each detector having atleast one GMR sensor coupled to an excitation circuit, each saiddetector having a mass of less than about 0.5 kg said detectorsoperating above about 70 °C., providing a sensitivity of better thanabout ±10 microOersted and a bandwidth from substantially DC to about 4MHZ and having sufficient vector sensitivity to provide a rejection ofat least about 30 dB; locating said plurality of sensors at a pluralityof locations on or below the earth's surface; measuring a magnetic fieldstrength at said plurality of locations, substantially simultaneously,using said plurality of sensors.